As Iowa State University Extension entomologists reported in an earlier article this summer, corn rootworm egg hatch in 2012 was slightly ahead of a normal year. Now the rootworms have emerged from the larvae (worm) stage and have developed into adult beetles. Adult corn rootworm beetles were first detected in Illinois last week and they've been seen flying around in corn fields in Iowa this week.

Farmers are asking—should they spray an insecticide to control adult corn rootworm beetles? Mainly, these farmers who are asking this question are worried about adult beetles feeding on the silks of corn plants and interfering with corn pollination.

Do You Need To Spray For Adult Corn Rootworm?

"We've been getting a lot of questions about when or if it is appropriate to spray cornfields to control the adult corn rootworm beetles," says ISU Extension entomologist Erin Hodgson. She and ISU research entomologist Aaron Gassmann offer the following information and explanation to help farmers make the right decision for the situation.

When is a foliar insecticide application warranted? Consider three factors

Three factors must be taken into account before a foliar insecticide application is warranted, say the ISU entomologists.

First, consider the growth stage of the corn plants. Adult corn rootworm beetles will feed on leaves and cause some scarring; however, this will have little if any effect on yield. Adults can cause yield loss if they are present in the field when corn is silking, which is a critical time period to protect plants. Adults are strongly attracted to silks and will gather in mass on the plants to feed and mate (see photo accompanying this article). Adults that feed on the silks and trim back silks to the husk during pollen shed in cornfields will interfere with pollination of the crop.

Second thing to consider, if you are trying to decide whether or not it will pay you to spray adult corn rootworm beetles, are the current growing conditions for the corn crop. "Weather plays a huge role on how corn plants respond to silk feeding by the adult beetles," says Hodgson. "Corn plants do not tolerate as much feeding during pollination in hot and dry weather."

For example, she says under ideal moisture conditions, plants could tolerate 15 beetles per plant, but that number is reduced to just five per plant under drought stress (Photo 2).

Finally, you need to consider corn plant maturity when deciding whether or not to spray for the adult beetles. Late-planted fields or late-flowering corn hybrids compared to neighboring fields are generally attractive to adult corn rootworm. Green silks will still be developing in these fields when older fields have brown or drying silks. Thus, the corn rootworm beetle adults may migrate and aggregate in these later-maturing fields.

Management recommendations: "It is always a smart idea to scout for insects in corn fields during pollination," says Hodgson. "But with adult corn rootworm becoming active before silking this year, we highly recommend keeping an eye on your corn fields to protect the yield. Spraying the field with a foliar insecticide may be warranted if there are five or more beetles per plant, if silks have been clipped to less than ½ inch of the ear tip, and pollination is not complete. Also take into consideration other insects that may be feeding on the silks at the same time (such as Japanese beetles."